1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
There is known a full-color image forming apparatus which forms color-separated images in black, yellow, cyan, and magenta on a plurality of photoconductors, respectively, and transfers each image on an intermediate transfer belt to be superposed. Among this kind of image forming apparatuses, there is an apparatus having two image forming functions, in other words, configurations of an image forming functional unit, one is for a monochrome page (a monochrome-only image forming function) and the other is for a color page (a color image forming function, a color mode). In the case of performing image formation of a monochrome page, the apparatus changes the image forming function from the color image forming function to the monochrome-only image forming function. For example, a position of the intermediate transfer belt is changed to put into contact only with a photoconductor for a black image so as to be transferable, but the intermediate transfer belt is not put into contact with the photoconductors for yellow, cyan, and magenta. By such a structure, abrasion of respective photoconductors for yellow, cyan, and magenta images can be suppressed. Further, the black image transferred on the intermediate transfer belt does not come into contact with the photoconductors for yellow, cyan, and magenta and therefore distortion of the black image can be prevented. In the case of forming color images, the position of the intermediate transfer belt is changed for color image forming and the intermediate transfer belt is put into contact with the respective photoconductors for black, yellow, cyan, and magenta so that the respective color images are transferred thereon.
However, in the aforementioned image forming apparatus, there is a case where a color page and a monochrome page are mixed during printing (printing job) in which a plurality of pages are included as one processing unit. In this case, if the image forming function is changed on a page by page basis, time for changing configuration is required. This lowers job efficiency. In view of reducing time loss due to position change of the intermediate transfer belt within one printing job, there is known a method which prints directly in a color mode even in the case of a monochrome page (for example, see Japanese Unexamined Patent Application Publication No. 2000-29266).
However, even if determination of the position change of the intermediate transfer belt is individually performed for each printing job, as in conventional way, efficient image forming is not necessarily realized, considering processing of a plurality of printing jobs. An example of such a case is where printing job as a copier (copy job) and printing job as a printer (print job) are alternatively processed.